1. Field of the Invention
The present invention relates to a digital radio communications receiver that predicts the structure of a received frame based on the phase and intervals of sync words picked up.
2. Description of the Related Art
In digital radio communications, correctly received information is extracted by detecting a received signal to extract received bit string and picking up a frame timing in the received bit string.
Typically, the detection of the frame timing and frame synchronization are performed by detecting a bit string that exhibits an outstanding autocorrelation in a predetermined position in frames, namely, by detecting a sync word.
A sync word is also referred to as a unique word, and the unique word is abbreviated UW in the drawings and the discussion that follows.
A unique word is detected by comparing a received bit string with the unique word bit string prepared at a receiver end. An unmatched bit count between both strings equal to or smaller than a predetermined threshold (hereinafter referred to as correlation threshold) determines that a unique word is detected.
On the other hand, an unmatched bit count at the timing of the unique word exceeds the correlation threshold determines that a unique word is missed.
When a frame synchronization is established, the receiver is capable of approximately predicting the position of the unique word. When the frame synchronization is established, a gate called an aperture is set up, and the probability of erroneous detection of the unique word is kept lowered by performing the valid detection of the unique word on or in the vicinity of the position of the unique word.
The frame synchronization is established by detecting the unique word at the predetermined positions for the specified number of consecutive frames. This operation is called backward guard and the specified number of frames is called the backward guard level. As the backward guard level is increased, the erroneous frame synchronization is less likely to take place, making higher the reliability of frame synchronization, but the time required to establish the frame synchronization gets longer. Conversely, as the guard level is decreased, the time required to establish the frame synchronization gets shorter but the erroneous frame synchronization is more likely to take place.
The detection of a frame missynchronzation is performed by verifying that the unique word is consecutively missed for the specified number of frames at the position where the unique word is supposed to appear. This operation is called forward guard, and the specified number of frames is called the forward guard level. As the forward guard level is increased, the probability of frame missynchronization due to degradation of channel quality or the like is reduced, but the time required to detect the frame missynchronization, when it actually takes place, is prolonged. Conversely, as the forward guard level is decreased, the time required to detect the frame missynchronization is shortened while the probability of the determination that a missynchronization is erroneously detected is heightened even in the situation where the frame synchronization needs to be maintained.
Some digital radio communications systems change the frame structure depending on communications conditions.
For example, in the system employing a voice activation technique, frames are transmitted only when voice remains significant, and no frames are in principle transmitted when no voice is recognized. In such a case, however, to maintain frame synchronization, a short burst containing a unique word is transmitted at regular intervals. Typically, this interval is different from the frame length.
When the frame structure changes depending on communications conditions as described above, a transmitter end is required to notify of the change in the frame structure. Available as methods of notifying of frame structure changes are one in which a predetermined bit string is set up for notifying of the frame structure in a frame and another method in which a bit string (hereinafter referred to as a frame structure flag) for notifying a change, when it takes place, is inserted.
FIG. 12 is a block diagram showing the configuration of a privately known but unpublished art digital radio communications receiver which performs frame synchronization and frame structure prediction.
Referring to FIG. 12, a unique word detector module 1 detects a unique word from a received bit string, based on the timing information from an aperture control module to be described later and the correlation threshold from a correlation threshold setting module to be described later. The radiowave received by a receiving antenna 100 is fed to a down-converter 101 which outputs a signal in an intermediate frequency bandwidth. A detector 102 detects the intermediate frequency signal and then outputs the received bit string to the unique word detector module 1. The aperture control module 2 outputs the timing information that controls the timing at which the unique word detector module 1 attempts to detect the unique word. In response to the aperture width from an aperture width setting module to be described later and the received timing information from a timing control module to be described later, the aperture control module 2 generates the timing information that is output to the unique word detector module 1.
There are further shown in FIG. 12 the timing control module 3 that outputs the receive timing information of the received signal in response to the unique word detection information from the unique word detector module 1, a frame synchronization guard level setting module 4 that sets frame synchronization determination conditions, namely, the backward guard level that is the number of consecutive detections of unique word and the forward guard level that is the number of consecutively misses of unique word (both levels are hereinafter collectively referred as the guard level), and a frame synchronization determining module 5 that results in the frame synchronization information, based on the unique word detection information from the unique word detector module 1 and the guard level from the frame synchronization guard level setting module 4.
There are yet further shown in FIG. 12 an aperture width setting module 6 that sets an aperture width as a time width within which the unique word detector module 1 attempts to detect a unique word, based on the unique word detection information from the unique word detector module 1 and the frame synchronization information from the frame synchronization determining module 5, a correlation threshold setting module 7 that sets the correlation threshold of unique word detection conditions, based on the unique word detection information from the unique word detector module 1 and the frame synchronization information from the frame synchronization determining module 5, a received signal extractor module 8 that extracts the received signal from the received bit string output by the detector 102 at the timing designated by the timing control module 3, and a frame structure determining module 9 for detecting the frame structure flag of the received signal to determine whether or not the frame structure changes.
The operation of the known digital radio communications receiver thus constructed is now discussed.
The radiowave received at the receiving antenna 100 is converted into an intermediate frequency signal, which is then fed, as a received signal by the down converter 101, to the detector 102. The detector 102 demodulates the received signal and outputs the received bit string.
The unique word detector module 1 receives the received bit string, correlates the received bit string with the unique word at the timing set by the aperture control module 2, detects the unique word and determines the phase of the unique word from the number of erratic bits and their correlation threshold, and then outputs the determination results as the unique word detection information.
The timing control module 3 controls the receive timing based on the unique word detection information.
The frame synchronization determining module 5 determines the frame synchronization state using the number of consecutive detections/misses of the unique word of the unique word detection information designated by the guard level setting module 4, and outputs the determination results as the frame synchronization information.
Referring to the unique word detection information and the frame synchronization information, the aperture width setting module 6 sets and outputs the aperture width that is used at the next attempt to detect the unique word.
Referring to the unique word detection information and the frame synchronization information, the correlation threshold setting module 7 sets and outputs the correlation threshold that is used at the next attempt to detect the unique word.
To determine whether the frame structure changes, the frame structure determining module 9 detects the frame structure flag indicative of the frame structure of the received signal that is extracted by the received signal extractor module 8 from the received bit string output by the detector 102 at the timing designated by the timing control module 3.
Discussed next is how the frame structure is recognized when the known art digital radio communications receiver performs frame synchronization control.
FIG. 13 shows an example of the change in the frame structure depending on communications conditions. Part of FIG. 13 herein shows a simplified version of FIG. 2 that is presented in a paper entitled "RADIO TRANSMISSION IN THE AMERICAN MOBILE SATELLITE SYSTEM" (A COLLECTION OF TECHNICAL PAPERS, AIAA-94-0945-CP, pp 280-294, 1994).
FIG. 13 shows a unique word 17, a frame structure flag 18-a indicative of a frame structure 1 and inserted at the change from a frame structure 2 to the frame structure 1, and a frame structure flag 18-b indicative of the frame structure 2 and inserted at the change from the frame structure 1 to the frame structure 2.
In the frame structure in FIG. 13, a unit or interval of the frame structure 1 delimited by unique words is called a subframe, and four subframes make up a frame. The interval between unique words in the frame structure 2 is identical to the frame length. In FIG. 13, in other words, the frame structure 1 has a unique word on a per subframe basis, and the frame structure 2 has a unique word on a per frame basis.
FIGS. 14 and 15 show examples of the recognition of the frame structure in which when the frame structure changes, a frame structure flag notifying of it is transmitted only once. FIG. 14 shows the example of the false detection of a frame structure flag, and FIG. 15 shows the example of a miss of a frame structure flag.
In FIG. 14, the frame structure determining module 9 suffers the false detection of a frame structure flag and thus erroneous determination of frame structure. The frame structure determining module 9 thus remains unable to receive a frame structure flag and thus unable to recognize correctly the frame structure until the frame structure is changed next.
In FIG. 15, the frame structure determining module 9 misses a frame structure flag and erroneously determines the frame structure. In this case, again, the frame structure determining module 9 remains unable to recognize correctly the frame structure until the next change in frame structure.
FIG. 16 shows an example of the effect of the above faulty determinations.
In the detection failure of the frame structure flag in FIG. 16, the frame synchronization forward guard level is 2.
As shown in FIG. 16, with the miss of the frame structure flag, the receiver attempts to receive the frame structure 1 though the frame is already changed from frame structure 1 to frame structure 2. Since the unique word interval is different between the frame structure 1 and the frame structure 2, the receiver suffers a detection failure of unique word in an attempt to detect the unique word with the unique word interval of the frame structure 1. Such a state continues until the frame is changed from frame structure 2 to frame structure 1, and it is highly likely that a missynchronization would take place in the course of repeated detection failures of the unique word.
In the known digital radio communication receiver thus constructed, when the flag notifying of the change in the frame structure is transmitted only once, followed by the failed or false detection of the flag, the receiver remains unable to correctly recognize the frame structure until the frame structure changes later again. Furthermore, the faulty recognition of the frame structure may cause the frame synchronization control to malfunction, possibly leading to a missynchronization.